The present invention generally relates to semiconductor device manufacturing, and more particularly to the fabrication of a heterojunction bipolar transistor (HBT) and field plate with improved break-down voltage.
The bipolar transistor is an electronic device with two p-n junctions in close proximity. The bipolar transistor has three device regions: an emitter, a collector, and a base disposed between the emitter and the collector. Ideally, the two p-n junctions (the emitter-base and collector-base junctions) are in a single layer of semiconductor material separated by a specific distance. Modulation of the current flow in one p-n junction by changing the bias of the nearby junction is called “bipolar-transistor action.”
External leads can be attached to each of the three regions and external voltages and currents can be applied to the device using these leads. If the emitter and collector are doped n-type and the base is doped p-type, the device is an “npn” transistor. Alternatively, if the opposite doping configuration is used, the device is a “pnp” transistor. Because the mobility of minority carriers (i.e., electrons) in the base region of npn transistors is higher than that of holes in the base of pnp transistors, higher-frequency operation and higher-speed performances can be obtained with npn devices. Therefore, npn transistors comprise the majority of bipolar transistors used to build integrated circuits.
The maximum (or cutoff) frequency of a pnp heterojunction bipolar transistor (HBT) is defined in part by the dopant concentration and thickness of a p-region between the n-type base and high doped p collector. As the dopant concentration of the p-region increases (or the thickness decreases), the collector resistance decreases, collector transit delay reduces and the cutoff frequency of the HBT increases. On the other hand, as the dopant concentration of the p-region decreases (or the thickness increases), the collector resistance increases, collector transit delay increases and the cutoff frequency of the HBT decreases.
The product of the breakdown voltage and the cutoff frequency produces a relatively constant value, which is commonly known as the Johnson limit. As the dopant concentration of the p-region increases, or the thickness reduces, the cutoff frequency of the HBT increases, while the breakdown voltage of the HBT decreases. On the other hand, as the dopant concentration of the p-region decreases or the thickness increases, the cutoff frequency of the HBT decreases while the breakdown voltage of HBT increases.
Similarly, the cutoff frequency of an npn HBT is defined in part by the dopant concentration and thickness of an n-region. Thus, as a result of the Johnson limit, as the dopant concentration of the n-region increases or the thickness reduces, the cutoff frequency of the HBT increases while the breakdown voltage of the HBT decreases. On the other hand, as the dopant concentration of n-region decreases or the thickness increases, the cutoff frequency of the HBT decreases while the breakdown voltage of HBT increases.